Methods of modulating a carrier with an information signal are generally known. Typically the modulation of the carrier with the information signal results in the frequency-shifting (translation) of the information signal between a base band and a spectral location of the carrier frequency. Frequency translation (up or down) may be accomplished by modulating one signal with another within a modulator (mixer).
Methods of modulating the carrier include amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM). While AM, FM and PAM are well known and reliable, they also transfer data at a low bit rate. Quadrature modulated signals, such as QPSK and QAM, have the capability of encoding each sample with a number of information bits and are, therefore, more efficient. For example, QPSK encode information at a rate of two bits per sample. One particular type of QAM (i.e., 16-QAM) encodes data at a rate of 4 bits per sample (symbol) period.
In order to encode data at a rate of 4 bits per sample, 16-QAM relies on a constellation of 16 symbols. Of the 16 symbols, each symbol differs from the other symbols of its constellation by a predetermined amplitude and phase.
In order to encode QAM signals, a modulator modulates a carrier with a phase and amplitude required by a particular symbol. Typically quadrature components (I and Q) of a local oscillator (LO) are provided as a first input to a modulator. A bit set (4-bits for 16-QAM) is provided as a second input of the modulator for each symbol period. Each bit combination of the bit set corresponds to a particular symbol of the constellation. A controller of the modulator reads each bit set and modulates the quadrature components of the LO to generate the appropriate amplitude and phase required for any particular symbol.
Within a receiver, the process may be reversed. To decode a quadrature signal, quadrature components of a local oscillator may be mixed with a received signal, filtered and detected. An estimator may be used to estimate the received symbol based upon the detected quadrature components.
While encoding an information signal using quadrature amplitude modulation is effective, the advent of newer technologies such as direct conversion, has made the process more difficult. In the case of direct downconversion, a QAM information signal (centered on FQAM) may be mixed with a LO signal (having a frequency fLO), of substantially the same frequency. While translation of the information signal to baseband may be effective using such a process, the process may also generate interfering signals. For example, mixing the information signal FQAM with the LO signal fLO causes the LO signal to be reflected back into the antenna. The LO may be reflected back into the antenna because the bandpass filter which isolates the desired information signal fQAM also passes the LO signal fLO. The reflection and interaction of the LO signal results in the generation of direct current (dc) interfering signals at baseband. The dc interference may result in a dc component within the decoded signal which is larger than the information signal, resulting in a significant decrease in a signal-to-noise S/N ratio. Because of the importance of QAM and direct conversion, a need exists for a direct conversion method which avoids generation of the dc signal components.